Given Pell's seniority in the Catholic Church as a former Vatican treasurer, the case is also of worldwide significance. The appeal involved complex legal principles. Here is what you need to know to understand the judgment.

What happened before this appeal?

In December 2018, a jury unanimously found Pell guilty of five sexual offences against two 13-year-old boys, committed while Archbishop of Melbourne. As detailed in the sentencing remarks of County Court Chief Judge Kidd in March 2019, Pell was found guilty of one count of sexual penetration of a child aged under 16 through forced oral sex, and four counts of an indecent act with or in the presence of a child aged under 16.

The first offences were committed in the sacristy of St Patrick's Cathedral after mass in December 1996. The final offence was committed against one of the boys around one month later. Both victims were choirboys and recipients of choral scholarships at an elite school.

In reaching a verdict, the jury relied on detailed evidence of one of the victims about what Pell said and did, and when and where it happened. The other victim began using heroin at age 14 and died of a heroin overdose in 2014, aged 31. This man's death prompted the surviving victim, aged in his early 30s, to approach police in 2015.

A 2013 study of 487 men whose mean age of onset of abuse was 10, found the mean age when first telling was 32.

Is it a problem that the prosecution relied on the complainant's evidence?

No. Child sexual abuse typically is inflicted in secret, without other evidence, so prosecutions often depend heavily on complainant testimony. The law recognises this: evidence does not have to be corroborated, and the judge must not warn the jury it is dangerous to act on uncorroborated evidence.

Juries make judgments based on the complainant account's credibility, consistency, detail and truthfulness, and responses and demeanour in cross-examination.

It also argued that it was not possible for Pell to have been in the sacristy either at all, or by himself; it was not possible for the boys to have been in the sacristy unnoticed; and the robes he wore made it impossible to offend in the way claimed.

What was the Court of Appeal required to do when considering this argument?

The law is complex, and whether a verdict is "unreasonable" depends on legal technicalities, not intuitive instincts. Four legal principles need to be understood here.

First, and most important, there is a very high threshold for a court to overturn a jury's guilty verdict for being unreasonable (see, for example, M or Baden-Clay). This is because, in Australian law, the jury is the constitutional tribunal of fact responsible for deciding guilt or innocence. A verdict will only be overturned in exceptional circumstances showing a clear miscarriage of justice.

Second, the test is whether, on the evidence, it was open to the jury to be satisfied beyond reasonable doubt the accused was guilty.

To win the appeal, the appellant must show the guilty verdict was not open to the jury. It is not sufficient for the court to find a jury might have had reasonable doubt. The evidence must mean no reasonable jury could have returned a guilty verdict; it must have "obliged" them to reach a not guilty verdict.

Third, the appeal court does not retry the case again, because the jury is the tribunal of fact. The court must independently assess the evidence, but to determine whether the guilty verdict was open to the jury; not simply whether the court itself has a doubt.

Fourth, if a complainant is credible and reliable and the account is detailed, consistent and plausible, it is difficult for an appeal to succeed. On plausibility, courts have accepted that sexual offending can be brazen, influenced by the abuser's arrogance, power and belief the child will not make a complaint.

What did the Court of Appeal say about this?

The judges rejected it by a majority of two to one. They found the guilty verdicts were reasonable, because they were open to the jury on the whole of the evidence.

The court said there was nothing about the evidence that meant the jury must have had reasonable doubt. It was not enough that one or more jurors might have had a doubt. Moreover, the court did not itself have such a doubt.

The complainant was found to be compelling, clearly not a liar or fantasist, and a witness of truth. He did not embellish the evidence or tailor it to the prosecution. He adequately explained things he could not remember and his explanations had a ring of truth.

What can happen now?

Pell can seek special leave to appeal to the High Court. If the High Court denies permission, the matter is finalised; if given, it will later deliver a final judgment.

Save for a successful appeal in the High Court, Pope Francis will likely expel Pell from the priesthood. The family of the second survivor is suing him and or the church for civil damages, as may others. Pell will remain in jail.

It is exceptionally difficult for survivors of child sexual abuse to bring successful criminal complaints, especially against powerful offenders. This judgment may encourage other courageous survivors to make complaints.

Yet many systemic reforms are still required to better facilitate prosecutions of child sexual offences.

(The writer Ben Mathews is Professor, School of Law, Queensland University of Technology).